Initial flu-like symptoms can lead to external haemorrhaging from areas like eyes and gums, and internal bleeding which can lead to organ failure.

Ebola kills up to 90% of those infected, with patients having a better chance of survival if they receive early treatment.

'A new level'

WHO Director General Margaret Chan is meeting West African presidents in the Guinean capital Conakry.

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The current outbreak is the deadliest since Ebola was discovered in 1976

"The scale of the Ebola outbreak, and the persistent threat it poses, requires WHO and Guinea, Liberia and Sierra Leone to take the response to a new level, and this will require increased resources, in-country medical expertise, regional preparedness and coordination," she said in a statement released on the WHO website on Thursday.

"The countries have identified what they need, and WHO is reaching out to the international community to drive the response plan forward."

Key elements of the WHO's new plan are:

Stopping transmission in the affected countries through "scaling up effective, evidence-based outbreak control measures"

Preventing the spread of Ebola to "the neighbouring at-risk countries through strengthening epidemic preparedness and response measures"

The response builds upon a previous plan that called for several hundred more personnel to be deployed to the region.

The WHO says that the scale of the ongoing outbreak is "unprecedented", with about 1,323 confirmed and suspected cases reported in Guinea, Liberia and Sierra Leone since March 2014.

A Samaritan's Purse medical worker demonstrates personal protective equipment to educate team members on the Ebola virus in Liberia

Liberian President Ellen Johnson Sirleaf has ordered all schools to be closed and put non-essential government workers on leave as part of a series of emergency measures to curtail the Ebola crisis

Office workers in Liberia have been advised to wear gloves as a protective measure to avoid the deadly Ebola virus

The plan is also expected to highlight the dangers faced by health workers on the front lines of the outbreak, and to call for improving ways to protect them from infection.

A spokeswoman for the US Centers for Disease Control and Prevention (CDC) said she was not aware of any Ebola patient ever being treated in the US before.

But in a statement the Atlanta hospital said it had an isolation unit which is specially equipped to deal with this kind of infection.

Dr Brantly's condition has deteriorated, but he insisted that a treatment serum be offered to his colleague

Meanwhile an American doctor with Ebola in Liberia has taken a "slight turn for the worse", the Samaritan's Purse aid agency said on Thursday.

Kent Brantly and another American worker, Nancy Writebol, "are in a stable but grave condition", the agency said in a statement.

The statement said that Dr Brantly had been offered experimental serum - using blood from a child whose life he saved - but he had insisted that Ms Writebol should receive it instead.

Health workers, doctors and nurses are a scarce resource in all three countries, and hundreds of international aid workers and more than a hundred WHO staff have already been deployed to support regional response efforts.

WHO says that improving prevention, detecting and reporting suspected cases, referring people infected with the disease for medical care, as well as psychosocial support, are of paramount importance in battling the illness.

Ebola since 1976

The WHO says that more emphasis need to be put on strengthening epidemic preparedness and response measures

The US is sending 50 extra specialists to affected areas.

In other developments:

President Ellen Johnson-Sirleaf of Liberia - one of the worst hit countries - told the BBC the Ebola outbreak was catastrophic, and more help was needed to contain its spread

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